A common misconception is that when you die, your daughter-in-law or son-in-law has the potential to claim a portion of your estate. The laws of inheritance in most states do not place in-laws in the category as statutory heirs or descendants. Descendants will, in most cases, include adopted children, but you didn’t adopt your in-laws just because your children married them. Most people do not want their in-laws to receive any portion of their estate. Not to worry! If you don’t write a will or trust naming them as beneficiaries, they will not get anything themselves.
However, if your child predeceases you and leaves minor children, your in-law, as those children’s parent, could end up with control over those funds. Therefore, if you have real concerns that your in-law would basically steal from their own children, then you need to expressly provide that under no circumstances shall that in-law (name them specifically) be appointed as a trustee or guardian of property or funds left by you in your will or trust.
Finally, some people actually WANT to include their in-laws. In those cases, it still may be important to you to add a qualifying clause, after you have expressly included them as beneficiary, that they will inherit only so long as they were married to my child at the time of your death. That technique will avoid having to change your estate plan in the event your child divorces.